As integrated circuit (IC) feature sizes shrink, problems of increased resistance and resistance-capacitance (RC) coupling offset any speed advantage derived from the smaller device size, limiting improvement in device performance. Ways to improve device performance and reliability include using highly conductive metals, such as copper, and employing lower dielectric constant (low-k) materials. The lower the dielectric constant of the dielectric, the lower the capacitance of the dielectric and the lower the RC delay of the IC.
Low k dielectrics are conventionally defined as those materials that have a dielectric constant lower than that of silicon dioxide, that is k<˜4. Typical methods of obtaining low-k materials include doping silicon dioxide with various hydrocarbons or fluorine. These doping methods, however, generally cannot produce materials with dielectric constants lower than about 2.6. With more and more advanced technology needs, present efforts are focused on developing low-k dielectric materials with k less than 2.5. These ultra low-k (ULK) dielectrics can be obtained by incorporating air voids within a low-k dielectric, creating a porous dielectric material.
Methods of fabricating porous dielectrics typically involve forming a composite film (sometimes referred to herein as a “precursor film”) containing two components: a porogen (typically an organic material such as a polymer) and a structure former or dielectric material (e.g., a silicon containing material). Once the composite film is formed on the substrate, the porogen component is removed, leaving a structurally intact porous dielectric matrix. Techniques for removing porogens from the composite film include, for example, a thermal process in which the substrate is heated to a temperature sufficient for the breakdown and vaporization of the organic porogen. These thermal processes, however, have certain difficulties. In particular, substrate temperatures generally need to be high (i.e., greater than about 400° C.) with exposure times typically on the order of hours. As is well known in the field, these conditions can damage copper containing devices.
Methods have been developed to form a porous low-k or ultra-low-k (ULK) film of dielectric material by first forming a precursor film that contains a porogen and a structure former, or “backbone”, on a substrate, and then exposing the precursor film to ultra-violet radiation (UV) in a curing process to remove the porogen. The method is disclosed in U.S. patent application Ser. No. 11/115,576 filed Apr. 26, 2005, issued as U.S. Pat. No. 8,137,465 on Mar. 20, 2012, and having the title “Single Chamber Sequential Curing of Semiconductor Wafers”, which is hereby incorporated by reference for all purposes.
The porogens evolved from low-k and ULK dielectric films in a UV curing chamber during a curing process tend to form porogen deposits on internal chamber parts and a quartz window through which ultraviolet curing (UVC) light is transmitted into a curing chamber. The resulting porogen deposit may become a source of particle contamination and a visual blemish. Deposits on the quartz window inhibit UVC light transmission and thus limit the number of wafers processed between chamber and window cleans.